Hilchos Sefiras Ha’omer – 6

1. Until the tenth day of the counting, we sayיָמִים,yamim, [e.g.,sheesha yamim] and from the eleventh day and on we sayיוֹם,yom [e.g.,achad asar yom]. Also, we say [in Hebrew] the smaller number first, such as: “Today is one-and-twenty days,” rather than, “twenty-one days.” These wordings are not critical but are only for the sake of clarity [in the Hebrew language] (Mishnah Berurah 489:9).

2. Even if one said the count of days using some other formula, such as on the thirty-ninth day he said, “Today is forty minus one,” he fulfilled his obligation (ibid.).